1. Field of the Invention
The present invention relates to an improved method and apparatus for mounting screens on frames. More specifically, the present invention relates to improved shapes of border constructions comprising cupping features to engage frames and with improved methods of mounting screens on frames for screen printing.
2. Prior Art
The concept of putting border constructions, usually embodied as rigid or semi-rigid strips of narrow width, along the edges of screen fabric so as to better attach fabric to a frame was perhaps first described by M. Louis Courtial in French patent No. 909,651 of 1945. Although this patent addressed fabric used in furnishings, it could equally have been applied to screen fabric stretched on printing frames. Other notable innovations are described in U.S. Pat. No. 2,903,967 by H.S. Levin in 1959, U.S. Pat. No. 3,078,793 by D. Jaffa et al in 1963, U.S. Pat. No. 3,211,089 by Elmar Messerschmitt in 1965, U.S. Pat. Nos. 5,274,934 and 5,522,314 by this applicant, Eugene F. Newman, Jr., in 1994 and 1996, respectively, U.S. Pat. No. 5,379,691 by Alan and Kaino Hamu in 1995, U.S. Pat. No. 5,390,596 by Gregory Farr in 1995 and U.S. Pat. No. 5,443,003 by James Larson in 1995.
The border strips of the above inventions are described as having many shapes to engage many types of frames. Newman, Jr., Farr, and Larson describe border strips, sometimes referred to as retainers or constructions, which are shaped so as to hook or cup over edges of a frame. The Newman Jr. invention comprises means to fasten border strips to external stretchers to hook or cup the border strips in place on a rigid frame while tensioning the screen. The Newman, Jr. invention, however, suffers from the problem that without further instruction, it would seem that there is no way to mount fabric on a rigid frame to its maximum tension when the fabric is first stretched onto the frame without using spacers. Also, it would seem that it would not be possible to mount the screen at maximum tension with less than four spacers per screen. Screen printers often prefer high tension as it is generally thought that this contributes to a better print. Since the hooking or cupping of border strips over edges of a frame requires an outward movement followed by an inward movement, the final inward movement actually undoes or reduces the tension in the screen. In the initial outward movements to clear the edges of a frame, if a screen reaches maximum tension before tearing the fabric, the subsequent inward hooking movements would, of course, reduce screen tension below the maximum. By pulling the border strips outwardly, spacers could be inserted between the frame and the border strips within the cupping features of the border strips so as to expand and hold the screen on the frame at maximum tension. However, four spacers would be needed. The placement of only two spacers to achieve maximum tension, one for the width and one for the length of the frame, would cause the cupping features to extend outwardly beyond the edges of the frame. This would unhook the border strips. Therefore, four spacers, one for each edge of the frame, must be inserted to achieve maximum tension unless specific modifications are made to the border strips. The present invention teaches modifications to U.S. Pat. No. 5,274,934 for mounting screens at maximum tension without needing spacers. Furthermore, the present invention teaches that as the screen inevitably losses tension over time, the screen can be re-tensioned with only one spacer required per axis, or a total of two rather than four spacers per screen. The present invention is intended as an improvement to U.S. Pat. No. 5,274,934.
In U.S. Pat. No. 5,522,314, a method is taught for pre-stretching screen fabric on a stretcher device and attaching border strips to the stretched fabric in a pattern that bears a relationship to the shape and size of a frame. This method allows the fabric and border strips to be later re-stretched onto a prescribed frame of a certain shape and size so as to achieve a prescribed tension in the screen. This method is much faster than an earlier method as described in Farr in which fabric is stretched and attached to a frame, a very slow and labor-intensive process, before the border strips (retainers) are attached onto the fabric in a pattern. However, the Newman, Jr. invention suffers from the necessity of having to stretch the fabric twice, once in a pre-stretch to attach the border strips and again to mount the bordered screen onto the frame. If the border strips are already cupped or hooked onto edges of the frame when the border strips are attached to the stretched fabric, the need to later mount the bordered screen onto the frame is eliminated. What is missing in this earlier art is the concept of using actual screen frames as fixtures upon which to mount and position the border strips. The present invention seeks to achieve this objective.
Hamu et al describes a frame having border strips (anchors) that are inserted into channels embedded in the print-side of a frame. Hamu, like Farr, then stretches and attaches fabric to a second frame. The second frame is then placed onto the print-side of the first frame having the border strips and the stretched fabric is bonded to the border strips within the channels. In this way, the stretched fabric is transferred onto border strips already mounted in the frame. This invention suffers from the same slowness and labor-intensity of the Farr invention inasmuch as it requires that the fabric be first stretched and attached to a second frame.
On a rigid frame with no moving parts, the Hamu border strips are designed to only work in channels and must rely upon an outer wall in the channel. On such a frame there is no way to re-tension the screen and there is no way to dislodge the Hamu border strips without destroying the fabric. The Hamu invention continues on to describe a frame constructed with internal stretcher devices added to provide re-tensioning, but such a frame is very expensive to construct and the screen suffers from a continued inability to be removed from the frame without destroying the screen. The teachings of Hamu U.S. Pat. No. 5,379,691 and Newman, Jr. U.S. Pat. No. 5,522,314 can be combined, but the resulting combination would not advance the art so as the avoid the disadvantages of the Hamu frame. An objective of the present invention is to teach an improvement to the method of U.S. Pat. No. 5,522,314 that works with any simple rigid frame with edges that can be hooked or cupped over. This would be almost all existing rigid frames. The present method produces a screen that can be re-tensioned and can be removed from a frame having no moving parts without destroying the screen.
In 1997 this applicant began to make and sell screens having polypropylene border strips that are heat sealed onto the fabric. These border strips, however, differ substantially from the present invention and are inoperable with the frames of the present invention. These border strips have a unique shape as described in U.S. Pat. No. 5,957,048 by this applicant. They are designed to go inside a channel with the bonded fabric inserted into the channel on the bottom side of the border strips and wrapped around a leg portion of the border strip so as to become lodged and pressed against a wall on the inside of the channel. The bonding of the fabric is on the bottom side of the border strips facing away from the print-side of the screen and oriented upside down in relation to the printing surface of the screen. These border strips will only work in channels. Also, since the bonding surfaces of these border strips become hidden inside the channels when they are mounted on the frame, there is no way they can be bonded to the fabric when mounted on the frame. As will be seen, this is a difference that makes these screens unworkable in the present invention. Yet another consideration is that the bonding of the fabric to the border strips of U.S. Pat. No. 5,957,048 was not designed to be the primary locking mechanism of the fabric on a frame. This bonding only had to be sufficient to hold the fabric on the border strips until the fabric became wedged between a leg portion of the border strips and inside walls of the channels of the frame. This required very little strength of bond. Strength was not a concern. The strength of bond between the border strips and the screen fabric in the screens made and sold by this author prior to 1999 were insufficiently strong to be used in the present invention. The present invention needs exceptionally strong bonds between the border strips and fabric because this is the only mechanism that holds the screen onto the frame.
Until the present invention, it has been difficult to find suitable materials to use for border strips of printing screens. Metal border strips are expensive and can only be glued to the screen. Wood is unacceptable because it tends to swell up when wet and this too must be glued. Most plastics are also unacceptable because they are attacked by solvents in printing inks. Also, most plastics will not melt at an acceptable temperature to be heat sealed to screens. Polypropylene, high density polyethylene, and other thermal plastics not only have high resistance to solvents, do not swell up when wet, but they also are capable of being melted at relatively low temperatures that will not harm most screen fabrics. These thermal plastics are relatively inexpensive materials and they can be very cheaply mass produced by injection molding and extruding. The possibility of using these materials, however, has not been recognized in the art except, as mentioned earlier, the applicant began to experiment and develop border strips in 1997 and before. Applicant was for many years frustrated with developing the extremely strong bonds needed to singularly hold a tightly stretched screen on a frame. The problem was to further develop these bonds so the border strips of the present invention and other border strips would perform well under high tension.
Objectives of the present invention include teaching improvements in the structure of border strips, teaching a new method of mounting these border strips on frames, and teaching an improved bond through heat sealing thermoplastic border strips onto screens.
The present invention comprises a rigid frame having a prescribed shape and size with four sides and no moving parts. It further comprises a rectangular screen fabric having rigid or semi-rigid border strips adhered to precisely measured locations along a portion of each edge of the screen fabric, not in the corner areas. The screen fabric and border strips are of a size and shape that bear a relationship with the size and shape of the frame. The screen fabric and border strips can be stretched onto the frame so as to achieve a prescribed tension in the screen, such as the recommended maximum safe tension for the fabric material before it tears. The border strips are hook-shaped or cupped so as to hook or cup onto the edges of a frame, holding the screen on the frame. The border strips can be pulled into place by hand or by external stretcher devices or in certain cases, as will be seen, the border strips may comprise internal stretcher devices. The elongated hook-like structures or cupping flanges of the border strips of the present invention may be of equal width on all border strips. However, in the preferred embodiment, two adjacent border strips comprise relatively wide cupping flanges and these are manually mounted first on the frame. The opposite adjacent border strips comprise cupping flanges of relatively narrow width, or in the alternative, as will be seen, these border strips may comprise removable sleeves which function much the same as flanges and which can slide off and on the border strips. Furthermore, the border strips may hook onto border connectors that, likewise, hook over the edges of a frame. Numerous embodiments of the present invention are possible.
In one embodiment of the present invention, border strips comprising adjacent relatively wide cupping flanges are manually mounted first and are opposite to border strips comprising relatively narrow cupping flanges. The border strips with relatively narrow cupping flanges can be pulled onto the frame by hand or by external stretchers, depending upon the degree of tightness sought. Because of the outward movement followed by an inward movement of the last two border strips, the screen ends up with less than maximum tension due to the final inward movement. The narrowness of the cupping flanges does minimize the loss of tension, however, assuming that it is intended that the screen be tightly stretched during the initial stretching. Because the cupping flanges on two adjacent border strips are relatively wide, spacers can easily be inserted into the gaps formed when these border strips are pulled away from the frame by an external stretcher device. The width of the relatively narrow cupping flanges is a distance the fabric relaxes when these border strips are first hooked or cupped and it is the minimum distance required to re-tension to reach maximum screen tension. If the widths of the cupping flanges of opposite border strips were the same, this would unhook either border strip, assuming that only one border strip is pulled outwardly. However, if the widths of the cupping flanges of the opposite border strips were unequal, the difference in their widths is the incremental distance beyond the minimum distance required to reach maximum screen tension before the relatively wide cupping flange becomes unhooked. This extra distance facilitates re-tensioning and permits one and not necessarily two of a pair of opposite border strips to be outwardly spaced on a frame so as to re-tension the screen up to maximum tension. Since there are two pairs of opposite border strips on a screen, this means two and not four border strips would need to be so spaced, a considerable savings in time and effort. If the border strips are on a relatively large piece of fabric in relation to the size of the frame, the difference in the widths of the cupping flanges must correspondingly be that much larger. Similarly, if the border strips are on a small piece of fabric in relation to the size of the frame, the difference in the widths of the cupping flanges can be correspondingly less. The size of the piece of fabric is determined by how much the fabric is stretched and the distance between border strips when the border strips are bonded to the fabric. A screen with a large difference in the widths of the cupping flanges would allow the screen to be stretched a large distance after mounting on a frame. If the screen were designed to tightly mount on the frame initially, less of an incremental stretch would be needed to bring the screen up to the maximum tension before tearing would occur.
All the border strips of the present invention may also comprise internal devices such as screws that could move these border strips to more outward positions from the frame edges. These could re-tension the screen without an external stretcher device.
In the above embodiment, the screen may have closely approached maximum tension in the initial stretching process, but it could not be mounted on the frame at maximum tension because of the final inward movement which undoes some of the tension. In an alternative embodiment, sleeves or caps that slide off and on the border strips along a tongue and groove joint serve as the cupping flanges. During the stretching process, the cap-sleeves are slid off the border strips. Stretcher devices pull all four border strips outwardly until the flat inside walls of the border strips align with the flat outside edges of the frame. At this point, the frame is inserted between all the border strips, the frame edges abut against the inside walls of the base of the border strips. The cap-sleeves are then put back on to the border strips. The cap-sleeves function like cupping flanges and hold the screen on the frame. Since there is no backward movement of the screen, it can be initially stretched all the way to maximum tension for mounting on the frame.
Although it would seem that the cap-sleeves need not have unequal cupping widths, the present invention comprises border strips with cap-sleeves having equal or unequal cupping widths. As will be later seen, such may be desirable to offer flexibility in how the screen is stretched on the frame initially and then later re-tensioned. Also, border strips having cap-sleeves may further comprise internal stretcher devices such as screws.
On larger frames, it may be necessary to have quite large border strips. If the strips are made of a disposable material, such as plastic, in which the material is permanently sealed onto the screen, disposing of the strips with every screen would be very wasteful. This is due to the amount of material involved. In order to reduce waste, it may be desirable to use border connectors that can hook or cup onto the large frame and which comprise edges over which relatively small border strips of disposable material can hook. The border connectors can be used over and over. These reusable border connectors can be shaped just like the various border strips described earlier. They may have permanent cupping flanges or they may have slide off and on cap-sleeves. They may have cupping flanges of equal or unequal widths as described above. The relatively small and inexpensive disposable border strips permanently attached to a screen hook onto the relatively large reusable border connectors which are hooked over the edges of the large printing frame. Since only the narrow lightweight strips are disposed of, there is much less waste than would otherwise be the case.
The border connectors described above may comprise means to attach to external stretcher devices or comprise internal stretching devices such as screws so as to retension the screen. Also, the border strips themselves, of course, as described earlier may comprise these features.
The present invention comprises border strips described above already adhered to fabric in precisely measured locations along each edge of the screen fabric, not in the corner areas. The present invention also comprises border strips as described above that are not adhered to fabric. The unattached border strips may be hooked or cupped over edges of frames so as to be mounted on the frames. Similarly, the unattached border strips may be hooked or cupped over the edges of connectors that are hooked or cupped over the edges of frames so as to mount the border strips and connectors on the frames. While so mounted, a single frame or a plurality of frames may have a sheet of fabric stretched by an external stretcher device with the fabric touching the top surfaces of the border strips. The fabric need not be stretched and mounted to an oversized frame but merely stretched with a stretcher device such as pneumatic clamps. With the fabric so stretched to a prescribed tension, the fabric is bonded to the frame mounted border strips. This completed, the external stretcher device can reduce the force exerted and the fabric held in place between the border strips on the frames will remain under high tension on the frames. Fabric outside the frame dimensions, which now is not tightly stretched, can be trimmed away and the frame or frames are ready for use. The above method saves time in mounting the bordered screens because the fabric is stretched only once, not twice, and the fabric is never mounted first on one frame and then on another frame. The above method can be used with all embodiments of the present invention. It can, likewise, be used with any existing frame that has edges suitable for hooking or cupping border strips and/or connectors. This would be almost any type of screen frame. The above method results in screens that are not permanently adhered to a frame and can be removed from the frame without destroying the screen. The screens can be re-tensioned. The frames do not have messy glue all over them when the screens are removed, so there is no clean up.
In the present invention, glue may be used to bond the fabric and border strips together. It is preferred, however, that the border strips be made of polypropylene or a similar material that can be heat sealed to the fabric. Polypropylene has a melting point much lower than most screen fabrics such as polyester. Experimentation has shown that polypropylene, if melted at 335 degrees Fahrenheit, will bond to polyester screen fabrics without harming the polyester screen. Border strips can be heat sealed to stretched fabric using an overhead plate that is powered so as to descend and ascend and comprises heat pads arranged to align and forcefully press onto screen fabric sandwiched between the heat pads and border strips. The border strips are mounted onto the edges of a frame or connectors on a frame. The border strips may also be mounted on a fixture under the stretched fabric in a pattern that bears a relationship to the size and shape of a prescribed frame. This process will produce bonds that are only minimally strong, however, because the polypropylene subtly sticks to the heat pads as they ascend, slightly pulling apart the fabric and polypropylene so as to leave a weakened bond. In the present invention, in order to achieve strong bonds, vellum paper or the like is sandwiched between the heat pads and the fabric. A stick resistant coating on the heat pads is also helpful. These steps taken, the subtle sticking is eliminated and the heating process produces surprisingly strong and consistent bonds.
A simple ironing device similar to those used to iron clothes can also heat seal the polypropylene border strips and fabric together. The heated iron is placed over the stretched fabric in contact with border strips that are mounted onto the edges of a frame or connectors on a frame. The border strips may also be mounted on a fixture under the stretched fabric in a pattern that bears a relationship to the size and shape of a prescribed frame. With the fabric sandwiched between the iron and the border strips, hand pressure is applied downwardly on the iron. In only a few seconds the polypropylene will melt adequately to bond to the fabric. By moving the iron along the paths of the border strips, the border strips and fabric are sealed together. If the iron has a very smooth bottom surface with a non-stick coating and/or if vellum paper is sandwiched between the iron and the fabric, sticking of the polypropylene to the iron is eliminated and again surprisingly strong seals are obtained. This process eliminates the fumes that are associated with gluing, it is fast, and it forms very strong bonds.